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H. N. Hutchinson
A Popular Account of Some of the Larger Forms of Ancient Animal Life
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But even creatures that had skeletons have not by any means always been preserved. Bones, when left on the bottom of the sea, where no sediment, or very little, is forming, will decay, and so disappear altogether. As Darwin points out, we are in error in supposing that over the greater part of the ocean-bed of the present day sediment is deposited fast enough to seal up organic remains before they can decay. Over a large part of the ocean-bed such cannot be the case; and this conclusion has, of late years, been confirmed by the observations made during the fruitful voyage of H.M.S. Challenger in the Atlantic and Pacific Oceans.
Again, even in shallower parts of the old seas, where sand or mud was once deposited, fossilisation was somewhat accidental; for some materials, being porous, allow of the percolation of water, and in this way shells, bones, etc., have been dissolved and lost. Thus sandstone strata are always barren in fossils compared to shales and limestones, which are much less pervious. To take examples from our own country, the New Red Sandstone of the south-west of England, the midland counties, Cheshire, and other parts contains very few fossils indeed, while the clays and limestones of the succeeding Lias period abound in organic remains of all sorts. Even insects have left delicate impressions of their wings and bodies! while shells, corals, encrinites, fish-teeth, and bones of saurians are found in great numbers.
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